The products of the SOS-regulated UmuDC operon are required for most of the mutagenesis caused by exposure to UV radiation or many chemicals. When the intact UmuD protein interacts with activated RecA, it undergoes a facilitated autodigestion that removes 24 amino acids from the N-terminus to generate UmuD'. This cleavage of UmuD activates it for its role in mutagenesis. We extended our analyses of our set of biologically active monocysteme derivatives of UmuD and UmuD' by constructing a set of monocysteine variants of UmuD and UmuD' in which cysteines were substituted at positions 133, 134, 135, 136, 137, and 138 (using UmuD numbering). Analyses of the abilities of these derivatives to crosslink by a disulfide bond after the addition of iodine supported the conclusion of NMR experiments [Ferentz et al., Nature Structural Biology 4:979-983 (1997)] that the C-termini of UmuD'2 are near each other in solution. Our observation of similar crosslinking with corresponding monocysteine derivatives of theUmuD2 homodimer suggest that the homodimer interfaces of UmuD'2 and UmuD2 share the common feature of having interacting C-termini. However our comparisons of the behavior of corresponding cysteines at positions such as 37, 38, and 57 of UmuD'2 versus UmuD2 had revealed that certain other features of molecular structure are very different between UmuD'2 and UmuD2. The identities of the disulfide-linked dimers were confirmed by mass spectrometry. Taken together these results have allowed us to formulate a model for how UmuD cleavage occurs and for the effects of cleavage upon the solution structure of the umuD gene product (manuscript in preparation).